


Birthday Heat

by I_Write_Sans_Not_Tragedies



Category: Undertale (Video Game)
Genre: Anal Sex, Begging, Bitty Bones, BittyBones, Blow Jobs, Dildos, Mating Cycles/In Heat, Multiple Orgasms, Overstimulation, Reader-Insert, Underswap Sans
Language: English
Status: Completed
Published: 2017-01-30
Updated: 2017-01-30
Packaged: 2018-09-21 00:06:44
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,042
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/9521945
Author URL: https://archiveofourown.org/users/I_Write_Sans_Not_Tragedies/pseuds/I_Write_Sans_Not_Tragedies
Summary: It's Blueberry's birthday, and you and your bitty have lots of fun things planned. Unfortunately, somebody forgot they were coming into heat, so the birthday plans take an unconventional turn.





	

**Author's Note:**

> this has been sitting around half-finished in my docs for AGES, figured I'd finish it up and post it. i'm a slut for bittybones porn >w>

 

You put the finishing touches on the three-layered cake and wiped your face. It looked absolutely awful, but the birthday boy had insisted on it being handmade.

A glance at the clock showed 7 in the morning- Blueberry was usually up by now, but he had waited until midnight to go to bed- so he could sleep for the first time now that he was… an undetermined age. Bitties didn’t really have ages. They didn’t have birthdays either, but you’d always celebrated it on the day you’d adopted him, this exact day three years ago. 

Baby Blue, or Blueberry, as you affectionately called him, had been excited about his birthday for weeks in advance. You’d wake up, eat his breakfast birthday cake, and then you had a day of amazing activities planned just for him. A visit to the mall to pick out presents, a detour to the nearest confectioner’s shop, a picnic in Blue’s favorite park- Blue had planned it all himself and you couldn’t be happier to indulge him.

You knocked the flour off of your shirt as you snuck back to your bedroom. You’d gotten up an hour earlier to decorate his cake- the two of you had made it together the night before. 

“Berry... you awake?” you called softly into the room. His little house was on a small table beside your bed. You walked over with a grin and peeked into the window, but he wasn’t there.

You heard a noise from underneath the bed.

“Oh, think I’m gonna let you scare me cause it’s your birthday?” you cooed, crouching down. “Well think again-”

Your bitty was sitting against one of the shoe boxes you kept underneath your bed, legs shifting, an insistent blue glow coming from his crotch. More importantly, though, he was crying. 

“... Berry?”

He burst into greater tears as he threw himself on your offered hand and bawled. He was hot and sweaty.

“I-I’m so- so sorry mama!” he sobbed. “I di-idn’t know I was gonna b-be in heat- now we can’t go to the mall or the park or anywhere, I ruined the whole day, I’m so so  _ so so sorry!”  _ he said, even as he began to grind on your hand. He stopped himself but he was still crying.

You scooped him up at once. “Shh, shh, oh honey, it’s alright,” you said. “It’s okay, Berry, you’re fine- these things can’t be helped.”

“B-But we were gonna- have so much fun! And I- I ruined the whole thing!” he buried his face in your shirt with a melodramatic whimper. “Now we can’t go  _ anywhere _ .”

You couldn’t help but smile at his dramatics. “We can do all that stuff tomorrow,” you said. “It isn’t your fault you’re in heat, baby.”

“Yes it is!” he cried, and you stopped yourself from giggling as he tossed and turned in your hands. “It’s all my fault, and tomorrow it won’t be the same cause it won’t be my birthday! The day I met my favorite human in the whole wide world!” He arched his back and squeaked out a frustrated groan.

You sighed and gave him a little kiss. “We can make your birthday special in other ways, okay? And then tomorrow we’ll go have all the fun there is.”

“It isn’t the same! Auuggh I hate being in heat!” he wailed. “It’s annoying and I get all dirty and  _ I can’t do anything while I’m in heat!” _

“Do you want me to help?” you asked, and he pulled his bandana up around most of his face. You’d only helped him a handful of times- only when they got particularly bad and he came to you with a boner and tears in his eyes. They didn’t make a lot of sex toys for bitties, which you thought was a crime given their inherent need for them, so he’d end up exhausted and cramped and still deep in heat. It wasn’t like he was embarrassed, he just took everything as a challenge. Dealing with a heat by himself was no exception.

“It doesn’t feel that bad,” he mumbled, averting his eyes. 

“I can still help, even if you don’t  _ need _ me to, baby. We can do whatever you want- it’s your birthday, remember?”

Blueberry turned an even brighter shade of cyan and grumbled to himself. You sighed and sat him down on the edge of your  bed, taking a knee beside it. He didn’t try to cover himself, just laid there and grumbled and shifted his legs.

You took the initiative and rubbed the front of his pants with your index finger.

“G-ah! Mama!” he scolded, and you giggled. Heats were nothing to be ashamed of- to monsters, they were like periods, kind of. Admittedly you would have loved to have insatiable libido for a day versus blood pouring out of your vagina, but hey, you could also open the fridge, so it was an even trade.

“You don’t wanna be alone on your  _ birthday _ , do you, Blue?” you asked, and you could see him smile past the edges of his little tiny bandana.

“Well I guess if you want to help that bad…” Blue said like he wasn’t the one with the giant boner and his soul shimmering through his ribs.

You helped him out of his shirt, which he flung at you with a gleeful little grin. You got him back by licking your thumb and running the pad along his sternum.

“O-Oh- not fair, human,” he said, but his eyes were hazy. You used both of your hands to touch his ribs, thumbing them while you reached the other fingers around and rubbed the backs. His little breathy moans told you he was ready for a little more.

“Tell me if it’s too much, okay?” you asked. “We can do whatever you want today.”

“C-Can you tie me up?” he asked.

You breathed a laugh as you ran your thumb along his spine. “Berry! I didn’t pin you as the kinky type.”

He arched his back into the touches. “A-Ah- please?”

You leaned forward and placed a kiss on his head before you pulled away from him and looked around the room.

It took a few minutes, but you figured it out. You got a piece of ribbon for his hands and did that first. You gently flipped him over and he complied readily. When his hands were secured behind his back you laid him back down and he shifted around, wiggling like a fat little worm.

“We should make a safeword,” you said, rubbing one of his cheeks. 

“What’s that?” he asked, legs shifting around the erection still tented in his sweatpants.

“You say it if you want me to stop and untie you.”

“What about… coffee?”

You laughed. “Coffee works for me.”

“W-Will… will you do my legs too?” he asked with a sheepish smile. 

You thought for a moment. Tying them together would make it hard for access… you snapped your fingers and scurried off to the kitchen.

You sewed, obviously, given how much Berry liked fashion and how expensive bitty clothes were from stores. You got two needles with the biggest end holes possible and taped them together, using a toothpick for further support in the middle. You picked up some thicker thread and hurried back to your little charge.

He really did look uncomfortable- his knees were drawn up as much as he could get them, moving around listlessly to try and stimulate his hard, aching little cock. When he saw you again, Berry relaxed.

You tied him up, looping the thick thread through the hole and the around his ankle. He seemed to realize what you were up to, and he blushed.

“You’re so creative, mama,” he said with a hint of mischief in his voice.

You finished with his other ankle and moved away to inspect your handywork. Yep. A functioning (albeit quite ugly) bitty spreader bar. 

Berry shifted his legs around, but he could no longer close them. He gulped and looked up at you with big, pleading eyes.

You didn’t waste a second. You ran your fingers over his ribs and spine and he gasped at the contact. 

You tugged his pants down and he yelped. His cock was so small- you thought it was pretty adorable. You ran the pad of your finger up along the bottom.

“Aaah… hh,” he groaned. “More, please,” he begged.

You kept up the touch. One hand traced his ribs, the other stroked up along his little dick. A bead of precum accumulated on the head and rolled down. You watched it curiously as he gasped and trembled from the touches.

You trailed the pad of your index finger around his tiny little head. With absolutely no warning his body seized up and he came with a sharp little cry. He  _ really _ must have needed it- his body jerked with each little twitch of his cock.

“A-Ah- ah-” he panted, staring down at his cock as it gave a last few desperate strains. You watched in fascination as the ejaculate seeped into his clothes. You continued to touch his ribs softly.

“You still good to go, Blue?” you asked him gently. His cock was still hard, cum dribbling down its length. Blue’s face was fully flushed, tears of overstimulation pricking his sockets.

“Yes- please keep g-going,” he stuttered, bucking his hips up into nothing and whining when there was nothing for his cock to rub against. You held back a little laugh and went back to work with your hands, this time curling your index finger against your thumb in order to make a small little hole. It took a bit of maneuvering and an angle that was not at all comfortable for you, but when you brought it down on him he immediately began bucking without mercy.

“A-Ah- ah- f-fuck-” he tittered fervently. You used your free hand to continue rubbing his sternum, the one curled into a make-shift opening pressing down to meet his wild, erratic thrusts. You had to be so careful with him- if you pressed down too hard you were afraid you’d break him. 

He continued to hump your hand. You were so enamored with his little movements and the hot feeling of his dick fucking your fingers that you didn’t notice him crying. His hips slowed until he was laying against the bed, panting and sweating and wriggling in his bonds.

“Mm- I can’t keep going,” he hiccuped, looking up at you with big teary eyes. 

You sighed through your smile. His cock twitched as you bent down and gave him a kiss on the side of his tiny little skull.

“I’ve got you,” you said sweetly. You started moving your hand again, faster this time, and he practically screamed. 

His body convulsed again as he came. His tossed and turned as much as he could in his position as you kept going, hand moving against him, paying special attention to the sensitive head. He came again within minutes, crying and panting and sobbing, cum slicking your hand. You grinned down at him.

"So sensitive..." you murmured.

“M-More- please mama I need more-” he begged when you finally slowed down. “Ah- please p… please put it i-in your mouth,” he cried, turning his head away as though embarrassed to say such things.

You hummed and promptly did as you were told. You wrapped your lips around the base of his little dick, tasting the tangy sweet magic that was coating the surface. He hissed as you swirled your tongue around it, careful not to put too much pressure on the little thing, and suckled. 

Blueberry yelped and thrust his hips up into the motion. You smiled as much as you could with his dick in your mouth and reached to the floor to pick up another little thing you’d brought with you. You were saving its debut for a special occasion, and this seemed special enough to you.

With a tiny little  _ pop _ you released his cock. He whined at the lack of stimulation but you quickly leaned down just a bit further, free hand looped underneath him to hold him up, and licked from his asshole to the tip of his dick in one long motion. He shivered and gasped.

“Mm- what’re you doing?” he asked, eyelights hazy and voice heavy. His shuddering breaths made you smile while you rubbed his length.

“You know that website we go on sometimes- the one with the weird dildos you laugh at?”

He sighed and relaxed into the blanket. “Good Dragon?”

You laughed. “Bad Dragon,” you corrected. He giggled until you rubbed around the head of his cock and he gasped.

“You know those little tiny ones…? The ones that are kind of like a gag thing?”

Blueberry leaned up at much as he could, his eyes wide with mischief. 

“Mama… you  _ didn’t _ …” he scolded, but his eyes were lit up with anticipation.

You grinned and reached down to pick one up. It was the smallest one you’d gotten, in the shape of some kind of fantasy creature, and appropriately colored blue.

Blueberry was practically  _ vibrating _ in excitement. “Oh my god! I can’t believe you actually bought them!”

“I thought you’d like it,” you said with a smile, reaching down for the lube you’d bought years ago and had forgotten you’d had at all. You lubed the thing up and put the head of it to Blue’s ass. He wriggled, and even though his excitement was tangible he looked nervous.

You leaned forward and kissed his skull. “Have you ever tried penetration before, baby?”

“N-No- will it hurt?” he asked softly. You rubbed the tip of the toy around his entrance and he looked up at you with big, scared eyes.

“We’ll go really slow,” you said softly. “It shouldn’t hurt, but if it does, just say so and we can stop.”

Blue nodded, eyes still locked on yours. He gulped and smiled.

“I love you,” he said quietly. You grinned and peppered his little skull with more smooches.

“I love you, too,” you said. 

It was an experience, working Blue’s asshole when you didn’t have anything smaller than a dildo to prepare him with. Luckily, with all the lube you put on and how relaxed and lost to his heat he was, you got the whole thing in in just a few minutes. 

He gritted his teeth, the little thing sat fully inside of him. You rubbed his skull with one hand and kept it in place with the other, his blue ectoflesh stretching to accommodate something so large.

“You okay, baby?”

“Y-Yeah- it feels… it feels weird,” he said, wiggling around on the tiny thing, cock bouncing in the process. “Good weird.”

You hummed, moving yourself back down to his cock. He hadn’t said anything, but it was twitching and his hips kept bucking with each movement. 

You lapped at it, sucking, gently humming around the length. He moaned and gasped in ecstasy, and you could feel his asshole throbbing around the fake cock. He shoulders shifted in vain, little white soul burning with magic and desire. He came again, all shaking bones and teary eyes and spasms. You kept sucking him off, playing with his ribs, and he didn't seem to mind one little bit.

You shallowly thrust the dildo into him and his whole body tensed with a cut-off cry. You were afraid you’d hurt him for a moment before his spine completely arched off of the bed.

“Fuck me- please f-fuck me-” he begged, beside himself with want, pressing himself down onto the cock as much as physically possible. “Please- oh my god-”

Who were you to deny your bitty? You continued sucking his cock as you shallowly thrust the dildo into his ass, taking it about halfway out before shoving it back into his shivering little frame. He cried out and sobbed, shifting his hips around and tossing his head. 

You fucked him hard. His whole body bounced with the force of it, cock twitching in desperation. 

“Fuck- o-oh fuck- I’m- I’m gonna- mama I’m gonna-!” he gasped and yelped as he came hard, seed spurting into your mouth. He jerked against his restraints with each squirt. You kept going, fucking him through it, watching his little soul shine bright for a few moments.

“A-Ah- ah- too much- c-coffee-” he sobbed. You stopped at once, removing your mouth and taking the dildo out slowly. He shivered from the feeling. It came out with a little  _ pop _ . He fell back against the bed with a shuddering breath, eyes shut tight and ribcage heaving with labored breaths.

“I’m sorry- are you okay?” you asked softly, reaching forward and enveloping him in both of your hands. His magic was beginning to dissipate, leaving him naked and shivering.

He hiccuped. “I-I’m okay. I think… I think it's over for now,” he said, looking up at you with big starry blue eyes. “Thank you for helping me,” he said quietly.

“Of course. Anything for my favorite little skeleton,” you said, giving him yet another kiss as you gently turned him onto his side and untied his hands, and then his legs shortly after. He stretched out his limbs, but he looked so so tired. You gathered him up and pressed him to your chest. He immediately curled up into a ball and clung onto you, letting out a deep, contented sigh.

“Sorry for ruining our day,” he said quietly into your shirt. “I can’t even return the favor…”

You rubbed his little back and shushed him. “No no, it’s fine. You’re fine, hun. Do you want a bath? You’re a little… sticky.”

He laughed and looked up at you. “You’re the one who got me all sticky, so… _you_ have to give me a bath,” he said with a cheeky little smile. 

You leaned down and nuzzled him. “You’re so bad… but I guess it  _ is _ your birthday, so just this once…”

He smiled and pressed himself against you. “I love you so much,” he whispered with a soft, kind little smile.

“I love you even more,” you replied, carrying him gently to the bathroom. 

It was an interesting birthday, to say the least.

**Author's Note:**

> [these are the tiny bad dragons i referenced](http://bad-dragon.com/products/teenieweenies)


End file.
